copy custom object properties to multiple objects
There is already a solution to copy custom-properties to and fro objects?
https://blender.stackexchange.com/a/24004/123064
import bpy

ob_sel = bpy.context.selected_editable_objects
ob_act = bpy.context.object

try:
    props = ob_act[&quot;_RNA_UI&quot;]
except KeyError:
    pass
else:
    for ob in ob_sel:
        if ob == ob_act:
            continue
        for p in props.keys():
            print(p, ob_act[p])
            ob[p] = ob_act[p]

Note: I am not sure if you are trying to create custom-properties additionally from a script or just trying to transfer them.